Schrödinger’s cat refers to a thought experiment proposed by the renowned Austrian physicist Erwin Schrödinger in 1935, who used it to illustrate the weirdness of quantum theory. Quantum theory says that until a particle is measured and observed, it actually exists in all the states it could possibly be in, simultaneously. So, if there is a statistical probability that this particle could be found here in one state, here in another state, here in another state and here in another state, then the particle will in fact be existing in all of those states at once. This is referred to as the particle being in a superposition.
When the particle is measured the superposition collapses and the particle chooses one state to be in overall of the others and this is the state that our measuring instrument will observe. If this is true then it should apply to larger objects since all matter is made up of particles. But of course, we know that that does not happen. We observe larger objects occupying only one state.
Schrödinger came up with the cat example to bring home just how odd the behavior of particles as described by quantum theory is. In this experiment, a cat is placed in a covered box with a radioactive sample that has a 50% chance of decaying and killing the cat in the next hour. While the box is covered, we have no idea if the cat is dead or alive and only once we open the box will we know if the cat made it or not.
So, if we think of the cat-like particle, using the superposition principle we would say that while the box was covered and the cat was not being observed, the cat was both dead and alive in order for it to be all the states it could possibly be in. Only when we lifted the box to observe the cat, that is once we carried out our act of measurement, did the cat’s superposition collapse for it to be either dead or alive.
Common sense tells us that before we uncovered the box the cat was either dead or alive but not both at the same time and our act of observing didn’t force one outcome over the other. But common sense is not something we can cling onto when studying the quantum realm. Countless experiments have shown us that sub-atomic particles behave in strange ways that we don’t observe in the larger matter.
There is a theory called the many worlds theory that states that anything that can happen does happen. So, when Schrödinger’s cat was observed in one universe the cat was found to be dead and in another universe the cat was found to be alive. The theory explains it like this: when the cat was in the covered box it was in the superposition of being both dead and alive. When the box was uncovered, a split in the Universe occurred where in one Universe the cat was dead and in another the cat was alive. The implications of this are mind-boggling. Every time there is more than one possible option available for the Universe, the Universe splits and creates copies where all possible outcomes come true.
So, for example there could be a world where Hitler won World War 2, a world where mankind never made it to the moon and a world where penicillin was never discovered. There could also theoretically be worlds that are much more advanced than us, a world where humans have been able to glimpse further and deeper into the workings of the Universe and have even been able to achieve immortality. If that was the case then every person has achieved quantum immortality in one alternate reality or another.